Revert "Remove unused GlobalUserPage::isSourcePage()"

Turns out now I want to use this function...oops.

This reverts commit e876c9e39f76f7e211c3c8b94dab433c358c25c3.

Change-Id: Ic365504be275c1ef278ee655674055e9e9c30d22
Merge "Revert "Remove unused GlobalUserPage::isSourcePage()""